Other than Earth, Wikipedia lists 17 planets that could have sunshine and liquid water in our solar system, at distances ranging from 4.2 lightyears to 2500 lightyears away. If this were a complete list, we could break down our solar system into habitable areas of 2500 lightyears in diameter.
We would further consider that life could not be sustained in the interior of the Milky Way, due to Gamma Rays, Black Holes and other DNA squashing regularities of the interior.
So this would leave around 200 quadrants around the edge of the Milky Way, with several more in the arms such as where the Earth resides. Since no life has been detected in our local quadrant, other than Earth, we have little information with which to fill in the variables of the Drake Equation. Perhaps most of these quadrants have no life. But others contain two, or three civilizations.
We don’t know. Perhaps our own quadrant contains life that is closer to bacteria than to human. What is the likelihood that there is some primitive bacterial, stromatolite or even sponge like life in our quadrant. Some might say the odds are high.
But the odds of another civilization in our Milky Way are still difficult to ascertain. Two more civilizations? Five more? Certainly we have enough data to think it is unlikely that there are hundreds, or even a dozen civilizations in the Milky Way, and we still have no proof that there must be more than one.
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